Network Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                    One Laptop per Child
Expires: May 20, 2009                                           J. Mogul
                                                              H. Frystyk
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                          T. Berners-Lee
                                                           Y. Lafon, Ed.
                                                         J. Reschke, Ed.
                                                       November 16, 2008

        HTTP/1.1, part 1: URIs, Connections, and Message Parsing

Status of this Memo

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   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 1 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 1 provides
   an overview of HTTP and its associated terminology, defines the
   "http" and "https" Uniform Resource Identifier (URI) schemes, defines
   the generic message syntax and parsing requirements for HTTP message
   frames, and describes general security concerns for implementations.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (  The current issues list is
   at <> and related
   documents (including fancy diffs) can be found at

   The changes in this draft are summarized in Appendix E.6.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Overall Operation  . . . . . . . . . . . . . . . . . . . .  6
   2.  Notational Conventions and Generic Grammar . . . . . . . . . .  8
     2.1.  ABNF Extension: #rule  . . . . . . . . . . . . . . . . . .  8
     2.2.  Basic Rules  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.3.  ABNF Rules defined in other Parts of the Specification . . 10
   3.  Protocol Parameters  . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  HTTP Version . . . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Uniform Resource Identifiers . . . . . . . . . . . . . . . 12
       3.2.1.  http URI scheme  . . . . . . . . . . . . . . . . . . . 13
       3.2.2.  URI Comparison . . . . . . . . . . . . . . . . . . . . 13
     3.3.  Date/Time Formats  . . . . . . . . . . . . . . . . . . . . 14
       3.3.1.  Full Date  . . . . . . . . . . . . . . . . . . . . . . 14
     3.4.  Transfer Codings . . . . . . . . . . . . . . . . . . . . . 16
       3.4.1.  Chunked Transfer Coding  . . . . . . . . . . . . . . . 17
     3.5.  Product Tokens . . . . . . . . . . . . . . . . . . . . . . 18
   4.  HTTP Message . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.1.  Message Types  . . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Message Headers  . . . . . . . . . . . . . . . . . . . . . 20
     4.3.  Message Body . . . . . . . . . . . . . . . . . . . . . . . 21
     4.4.  Message Length . . . . . . . . . . . . . . . . . . . . . . 22
     4.5.  General Header Fields  . . . . . . . . . . . . . . . . . . 23
   5.  Request  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Request-Line . . . . . . . . . . . . . . . . . . . . . . . 24
       5.1.1.  Method . . . . . . . . . . . . . . . . . . . . . . . . 24
       5.1.2.  Request-URI  . . . . . . . . . . . . . . . . . . . . . 24
     5.2.  The Resource Identified by a Request . . . . . . . . . . . 26
   6.  Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     6.1.  Status-Line  . . . . . . . . . . . . . . . . . . . . . . . 27
       6.1.1.  Status Code and Reason Phrase  . . . . . . . . . . . . 27
   7.  Connections  . . . . . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  Persistent Connections . . . . . . . . . . . . . . . . . . 28
       7.1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . 28
       7.1.2.  Overall Operation  . . . . . . . . . . . . . . . . . . 28
       7.1.3.  Proxy Servers  . . . . . . . . . . . . . . . . . . . . 30
       7.1.4.  Practical Considerations . . . . . . . . . . . . . . . 30
     7.2.  Message Transmission Requirements  . . . . . . . . . . . . 31
       7.2.1.  Persistent Connections and Flow Control  . . . . . . . 31
       7.2.2.  Monitoring Connections for Error Status Messages . . . 31
       7.2.3.  Use of the 100 (Continue) Status . . . . . . . . . . . 32
       7.2.4.  Client Behavior if Server Prematurely Closes
               Connection . . . . . . . . . . . . . . . . . . . . . . 34
   8.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 34
     8.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.2.  Content-Length . . . . . . . . . . . . . . . . . . . . . . 36

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     8.3.  Date . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
       8.3.1.  Clockless Origin Server Operation  . . . . . . . . . . 37
     8.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.5.  TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.6.  Trailer  . . . . . . . . . . . . . . . . . . . . . . . . . 39
     8.7.  Transfer-Encoding  . . . . . . . . . . . . . . . . . . . . 40
     8.8.  Upgrade  . . . . . . . . . . . . . . . . . . . . . . . . . 41
     8.9.  Via  . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 43
     9.1.  Message Header Registration  . . . . . . . . . . . . . . . 43
     9.2.  URI Scheme Registration  . . . . . . . . . . . . . . . . . 44
     9.3.  Internet Media Type Registrations  . . . . . . . . . . . . 44
       9.3.1.  Internet Media Type message/http . . . . . . . . . . . 44
       9.3.2.  Internet Media Type application/http . . . . . . . . . 45
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 46
     10.1. Personal Information . . . . . . . . . . . . . . . . . . . 47
     10.2. Abuse of Server Log Information  . . . . . . . . . . . . . 47
     10.3. Attacks Based On File and Path Names . . . . . . . . . . . 47
     10.4. DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . . 47
     10.5. Proxies and Caching  . . . . . . . . . . . . . . . . . . . 48
     10.6. Denial of Service Attacks on Proxies . . . . . . . . . . . 49
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 49
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 50
     12.2. Informative References . . . . . . . . . . . . . . . . . . 51
   Appendix A.  Tolerant Applications . . . . . . . . . . . . . . . . 53
   Appendix B.  Conversion of Date Formats  . . . . . . . . . . . . . 54
   Appendix C.  Compatibility with Previous Versions  . . . . . . . . 54
     C.1.  Changes from HTTP/1.0  . . . . . . . . . . . . . . . . . . 55
       C.1.1.  Changes to Simplify Multi-homed Web Servers and
               Conserve IP Addresses  . . . . . . . . . . . . . . . . 55
     C.2.  Compatibility with HTTP/1.0 Persistent Connections . . . . 56
     C.3.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 57
     C.4.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 57
   Appendix D.  Terminology . . . . . . . . . . . . . . . . . . . . . 58
   Appendix E.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 61
     E.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 61
     E.2.  Since draft-ietf-httpbis-p1-messaging-00 . . . . . . . . . 61
     E.3.  Since draft-ietf-httpbis-p1-messaging-01 . . . . . . . . . 62
     E.4.  Since draft-ietf-httpbis-p1-messaging-02 . . . . . . . . . 63
     E.5.  Since draft-ietf-httpbis-p1-messaging-03 . . . . . . . . . 64
     E.6.  Since draft-ietf-httpbis-p1-messaging-04 . . . . . . . . . 64
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 69
   Intellectual Property and Copyright Statements . . . . . . . . . . 72

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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is an application-level
   request/response protocol that uses extensible semantics and MIME-
   like message payloads for flexible interaction with network-based
   hypermedia information systems.  HTTP relies upon the Uniform
   Resource Identifier (URI) standard [RFC3986] to indicate resource
   targets for interaction and to identify other resources.  Messages
   are passed in a format similar to that used by Internet mail
   [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
   [RFC2045] (see Appendix A of [Part3] for the differences between HTTP
   and MIME messages).

   HTTP is also designed for use as a generic protocol for translating
   communication to and from other Internet information systems, such as
   USENET news services via NNTP [RFC3977], file services via FTP
   [RFC959], Gopher [RFC1436], and WAIS [WAIS].  HTTP proxies and
   gateways provide access to alternative information services by
   translating their diverse protocols into a hypermedia format that can
   be viewed and manipulated by clients in the same way as HTTP

   This document is Part 1 of the seven-part specification of HTTP,
   defining the protocol referred to as "HTTP/1.1" and obsoleting
   [RFC2616].  Part 1 defines how clients determine when to use HTTP,
   the URI schemes specific to HTTP-based resources, overall network
   operation with transport protocol connection management, and HTTP
   message framing.  Our goal is to define all of the mechanisms
   necessary for HTTP message handling that are independent of message
   semantics, thereby defining the complete set of requirements for an
   HTTP message relay or generic message parser.

1.1.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements for the protocols it
   implements.  An implementation that satisfies all the MUST or
   REQUIRED level and all the SHOULD level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the MUST level requirements but not all the SHOULD
   level requirements for its protocols is said to be "conditionally

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1.2.  Overall Operation

   HTTP is a request/response protocol.  A client sends a request to the
   server in the form of a request method, URI, and protocol version,
   followed by a MIME-like message containing request modifiers, client
   information, and possible body content over a connection with a
   server.  The server responds with a status line, including the
   message's protocol version and a success or error code, followed by a
   MIME-like message containing server information, entity
   metainformation, and possible entity-body content.  The relationship
   between HTTP and MIME is described in Appendix A of [Part3].

   Most HTTP communication is initiated by a user agent and consists of
   a request to be applied to a resource on some origin server.  In the
   simplest case, this may be accomplished via a single connection (v)
   between the user agent (UA) and the origin server (O).

          request chain ------------------------>
       UA -------------------v------------------- O
          <----------------------- response chain

   A more complicated situation occurs when one or more intermediaries
   are present in the request/response chain.  There are three common
   forms of intermediary: proxy, gateway, and tunnel.  A proxy is a
   forwarding agent, receiving requests for a URI in its absolute form,
   rewriting all or part of the message, and forwarding the reformatted
   request toward the server identified by the URI.  A gateway is a
   receiving agent, acting as a layer above some other server(s) and, if
   necessary, translating the requests to the underlying server's
   protocol.  A tunnel acts as a relay point between two connections
   without changing the messages; tunnels are used when the
   communication needs to pass through an intermediary (such as a
   firewall) even when the intermediary cannot understand the contents
   of the messages.

          request chain -------------------------------------->
       UA -----v----- A -----v----- B -----v----- C -----v----- O
          <------------------------------------- response chain

   The figure above shows three intermediaries (A, B, and C) between the
   user agent and origin server.  A request or response message that
   travels the whole chain will pass through four separate connections.
   This distinction is important because some HTTP communication options
   may apply only to the connection with the nearest, non-tunnel
   neighbor, only to the end-points of the chain, or to all connections
   along the chain.  Although the diagram is linear, each participant
   may be engaged in multiple, simultaneous communications.  For
   example, B may be receiving requests from many clients other than A,

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   and/or forwarding requests to servers other than C, at the same time
   that it is handling A's request.

   Any party to the communication which is not acting as a tunnel may
   employ an internal cache for handling requests.  The effect of a
   cache is that the request/response chain is shortened if one of the
   participants along the chain has a cached response applicable to that
   request.  The following illustrates the resulting chain if B has a
   cached copy of an earlier response from O (via C) for a request which
   has not been cached by UA or A.

             request chain ---------->
          UA -----v----- A -----v----- B - - - - - - C - - - - - - O
             <--------- response chain

   Not all responses are usefully cacheable, and some requests may
   contain modifiers which place special requirements on cache behavior.
   HTTP requirements for cache behavior and cacheable responses are
   defined in Section 1 of [Part6].

   In fact, there are a wide variety of architectures and configurations
   of caches and proxies currently being experimented with or deployed
   across the World Wide Web. These systems include national hierarchies
   of proxy caches to save transoceanic bandwidth, systems that
   broadcast or multicast cache entries, organizations that distribute
   subsets of cached data via CD-ROM, and so on.  HTTP systems are used
   in corporate intranets over high-bandwidth links, and for access via
   PDAs with low-power radio links and intermittent connectivity.  The
   goal of HTTP/1.1 is to support the wide diversity of configurations
   already deployed while introducing protocol constructs that meet the
   needs of those who build web applications that require high
   reliability and, failing that, at least reliable indications of

   HTTP communication usually takes place over TCP/IP connections.  The
   default port is TCP 80
   (<>), but other ports can
   be used.  This does not preclude HTTP from being implemented on top
   of any other protocol on the Internet, or on other networks.  HTTP
   only presumes a reliable transport; any protocol that provides such
   guarantees can be used; the mapping of the HTTP/1.1 request and
   response structures onto the transport data units of the protocol in
   question is outside the scope of this specification.

   In HTTP/1.0, most implementations used a new connection for each
   request/response exchange.  In HTTP/1.1, a connection may be used for
   one or more request/response exchanges, although connections may be
   closed for a variety of reasons (see Section 7.1).

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2.  Notational Conventions and Generic Grammar

2.1.  ABNF Extension: #rule

   One extension to the ABNF rules of [RFC5234] is used to improve

   A construct "#" is defined, similar to "*", for defining lists of
   elements.  The full form is "<n>#<m>element" indicating at least <n>
   and at most <m> elements, each separated by one or more commas (",")
   and OPTIONAL linear white space (OWS).  This makes the usual form of
   lists very easy; a rule such as

    ( *OWS element *( *OWS "," *OWS element ))

   can be shown as


   Wherever this construct is used, null elements are allowed, but do
   not contribute to the count of elements present.  That is,
   "(element), , (element) " is permitted, but counts as only two
   elements.  Therefore, where at least one element is required, at
   least one non-null element MUST be present.  Default values are 0 and
   infinity so that "#element" allows any number, including zero;
   "1#element" requires at least one; and "1#2element" allows one or

   [[abnf.list: At a later point of time, we may want to add an appendix
   containing the whole ABNF, with the list rules expanded to strict RFC
   5234 notation.]]

2.2.  Basic Rules

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].  The following core rules are included by
   reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
   CHAR (any [USASCII] character, excluding NUL), CR (carriage return),
   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
   (line feed), OCTET (any 8-bit sequence of data), SP (space) and WSP
   (white space).

   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
   protocol elements except the entity-body (see Appendix A for tolerant
   applications).  The end-of-line marker within an entity-body is
   defined by its associated media type, as described in Section 3.3 of

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   All linear white space (LWS) in header field-values has the same
   semantics as SP.  A recipient MAY replace any such linear white space
   with a single SP before interpreting the field value or forwarding
   the message downstream.

   Historically, HTTP/1.1 header field values allow linear white space
   folding across multiple lines.  However, this specification
   deprecates its use; senders MUST NOT produce messages that include
   LWS folding (i.e., use the obs-fold rule), except within the message/
   http media type (Section 9.3.1).  Receivers SHOULD still parse folded
   linear white space.

   This specification uses three rules to denote the use of linear white
   space; BWS ("Bad" White Space), OWS (Optional White Space), and RWS
   (Required White Space).

   "Bad" white space is allowed by the BNF, but senders SHOULD NOT
   produce it in messages.  Receivers MUST accept it in incoming

   Required white space is used when at least one linear white space
   character is required to separate field tokens.  In all such cases, a
   single SP character SHOULD be used.

     OWS            = *( [ obs-fold ] WSP )
                    ; "optional" white space
     RWS            = 1*( [ obs-fold ] WSP )
                    ; "required" white space
     BWS            = OWS
                    ; "bad" white space
     obs-fold       = CRLF

   The TEXT rule is only used for descriptive field contents and values
   that are not intended to be interpreted by the message parser.  Words
   of *TEXT MAY contain characters from character sets other than ISO-
   8859-1 [ISO-8859-1] only when encoded according to the rules of

     TEXT           = %x20-7E / %x80-FF / OWS
                    ; any OCTET except CTLs, but including OWS

   A CRLF is allowed in the definition of TEXT only as part of a header
   field continuation.  It is expected that the folding LWS will be
   replaced with a single SP before interpretation of the TEXT value.

   Many HTTP/1.1 header field values consist of words separated by LWS
   or special characters.  These special characters MUST be in a quoted

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   string to be used within a parameter value (as defined in
   Section 3.4).

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA

     token          = 1*tchar

   Comments can be included in some HTTP header fields by surrounding
   the comment text with parentheses.  Comments are only allowed in
   fields containing "comment" as part of their field value definition.
   In all other fields, parentheses are considered part of the field

     comment        = "(" *( ctext / quoted-pair / comment ) ")"
     ctext          = <any TEXT excluding "(" and ")">

   A string of text is parsed as a single word if it is quoted using
   double-quote marks.

     quoted-string  = DQUOTE *(qdtext / quoted-pair ) DQUOTE
     qdtext         = <any TEXT excluding DQUOTE and "\">

   The backslash character ("\") MAY be used as a single-character
   quoting mechanism only within quoted-string and comment constructs.

     quoted-text    = %x01-09 /
                      %x0B-0C /
                      %x0E-FF ; Characters excluding NUL, CR and LF
     quoted-pair    = "\" quoted-text

2.3.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     request-header  = <request-header, defined in [Part2], Section 4>
     response-header = <response-header, defined in [Part2], Section 6>

     accept-params   = <accept-params, defined in [Part3], Section 6.1>
     entity-body     = <entity-body, defined in [Part3], Section 4.2>
     entity-header   = <entity-header, defined in [Part3], Section 4.1>

     Cache-Control   = <Cache-Control, defined in [Part6], Section 16.4>
     Pragma          = <Pragma, defined in [Part6], Section 16.4>
     Warning         = <Warning, defined in [Part6], Section 16.6>

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3.  Protocol Parameters

3.1.  HTTP Version

   HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
   of the protocol.  The protocol versioning policy is intended to allow
   the sender to indicate the format of a message and its capacity for
   understanding further HTTP communication, rather than the features
   obtained via that communication.  No change is made to the version
   number for the addition of message components which do not affect
   communication behavior or which only add to extensible field values.
   The <minor> number is incremented when the changes made to the
   protocol add features which do not change the general message parsing
   algorithm, but which may add to the message semantics and imply
   additional capabilities of the sender.  The <major> number is
   incremented when the format of a message within the protocol is
   changed.  See [RFC2145] for a fuller explanation.

   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message.  HTTP-Version is case-sensitive.

     HTTP-Version   = HTTP-Prot-Name "/" 1*DIGIT "." 1*DIGIT
     HTTP-Prot-Name = %x48.54.54.50 ; "HTTP", case-sensitive

   Note that the major and minor numbers MUST be treated as separate
   integers and that each MAY be incremented higher than a single digit.
   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
   lower than HTTP/12.3.  Leading zeros MUST be ignored by recipients
   and MUST NOT be sent.

   An application that sends a request or response message that includes
   HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
   with this specification.  Applications that are at least
   conditionally compliant with this specification SHOULD use an HTTP-
   Version of "HTTP/1.1" in their messages, and MUST do so for any
   message that is not compatible with HTTP/1.0.  For more details on
   when to send specific HTTP-Version values, see [RFC2145].

   The HTTP version of an application is the highest HTTP version for
   which the application is at least conditionally compliant.

   Proxy and gateway applications need to be careful when forwarding
   messages in protocol versions different from that of the application.
   Since the protocol version indicates the protocol capability of the
   sender, a proxy/gateway MUST NOT send a message with a version
   indicator which is greater than its actual version.  If a higher
   version request is received, the proxy/gateway MUST either downgrade
   the request version, or respond with an error, or switch to tunnel

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   Due to interoperability problems with HTTP/1.0 proxies discovered
   since the publication of [RFC2068], caching proxies MUST, gateways
   MAY, and tunnels MUST NOT upgrade the request to the highest version
   they support.  The proxy/gateway's response to that request MUST be
   in the same major version as the request.

      Note: Converting between versions of HTTP may involve modification
      of header fields required or forbidden by the versions involved.

3.2.  Uniform Resource Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used in HTTP to
   indicate the target of a request and to identify additional resources
   related to that resource, the request, or the response.  Each
   protocol element in HTTP that allows a URI reference will indicate in
   its ABNF whether the element allows only a URI in absolute form, any
   relative reference, or some limited subset of the URI-reference
   grammar.  Unless otherwise indicated, relative URI references are to
   be parsed relative to the URI corresponding to the request target
   (the base URI).

   This specification adopts the definitions of "URI-reference",
   "absolute-URI", "fragment", "port", "host", "path-abempty", "path-
   absolute", "query", and "authority" from [RFC3986]:

     absolute-URI   = <absolute-URI, defined in [RFC3986], Section 4.3>
     authority     = <authority, defined in [RFC3986], Section 3.2>
     fragment      = <fragment, defined in [RFC3986], Section 3.5>
     path-abempty  = <path-abempty, defined in [RFC3986], Section 3.3>
     path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
     port          = <port, defined in [RFC3986], Section 3.2.3>
     query         = <query, defined in [RFC3986], Section 3.4>
     uri-host      = <host, defined in [RFC3986], Section 3.2.2>

     relative-part = <relative-part, defined in [RFC3986], Section 4.2>
     relativeURI   = relative-part [ "?" query ]

   HTTP does not place an a priori limit on the length of a URI.
   Servers MUST be able to handle the URI of any resource they serve,
   and SHOULD be able to handle URIs of unbounded length if they provide
   GET-based forms that could generate such URIs.  A server SHOULD
   return 414 (Request-URI Too Long) status if a URI is longer than the
   server can handle (see Section 9.4.15 of [Part2]).

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      Note: Servers ought to be cautious about depending on URI lengths
      above 255 bytes, because some older client or proxy
      implementations might not properly support these lengths.

3.2.1.  http URI scheme

   The "http" scheme is used to locate network resources via the HTTP
   protocol.  This section defines the syntax and semantics for
   identifiers using the http or https URI schemes.

     http-URI = "http:" "//" authority path-abempty [ "?" query ]

   If the port is empty or not given, port 80 is assumed.  The semantics
   are that the identified resource is located at the server listening
   for TCP connections on that port of that host, and the Request-URI
   for the resource is path-absolute (Section 5.1.2).  The use of IP
   addresses in URLs SHOULD be avoided whenever possible (see
   [RFC1900]).  If the path-absolute is not present in the URL, it MUST
   be given as "/" when used as a Request-URI for a resource
   (Section 5.1.2).  If a proxy receives a host name which is not a
   fully qualified domain name, it MAY add its domain to the host name
   it received.  If a proxy receives a fully qualified domain name, the
   proxy MUST NOT change the host name.

      Note: the "https" scheme is defined in [RFC2818].

3.2.2.  URI Comparison

   When comparing two URIs to decide if they match or not, a client
   SHOULD use a case-sensitive octet-by-octet comparison of the entire
   URIs, with these exceptions:

   o  A port that is empty or not given is equivalent to the default
      port for that URI-reference;

   o  Comparisons of host names MUST be case-insensitive;

   o  Comparisons of scheme names MUST be case-insensitive;

   o  An empty path-absolute is equivalent to an path-absolute of "/".

   Characters other than those in the "reserved" set (see [RFC3986],
   Section 2.2) are equivalent to their ""%" HEXDIG HEXDIG" encoding.

   For example, the following three URIs are equivalent:

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3.3.  Date/Time Formats

3.3.1.  Full Date

   HTTP applications have historically allowed three different formats
   for the representation of date/time stamps:

      Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
      Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
      Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

   The first format is preferred as an Internet standard and represents
   a fixed-length subset of that defined by [RFC1123] (an update to
   [RFC822]).  The other formats are described here only for
   compatibility with obsolete implementations.  HTTP/1.1 clients and
   servers that parse the date value MUST accept all three formats (for
   compatibility with HTTP/1.0), though they MUST only generate the RFC
   1123 format for representing HTTP-date values in header fields.  See
   Appendix A for further information.

      Note: Recipients of date values are encouraged to be robust in
      accepting date values that may have been sent by non-HTTP
      applications, as is sometimes the case when retrieving or posting
      messages via proxies/gateways to SMTP or NNTP.

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception.  For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time).  This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format.  HTTP-date is case sensitive and MUST NOT include
   additional LWS beyond that specifically included as SP in the

     HTTP-date    = rfc1123-date / obsolete-date
     obsolete-date = rfc850-date / asctime-date
     rfc1123-date = wkday "," SP date1 SP time SP GMT
     rfc850-date  = weekday "," SP date2 SP time SP GMT
     asctime-date = wkday SP date3 SP time SP 4DIGIT
     date1        = 2DIGIT SP month SP 4DIGIT
                    ; day month year (e.g., 02 Jun 1982)
     date2        = 2DIGIT "-" month "-" 2DIGIT
                    ; day-month-year (e.g., 02-Jun-82)
     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                    ; month day (e.g., Jun  2)
     time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT

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                    ; 00:00:00 - 23:59:59
     wkday        = s-Mon / s-Tue / s-Wed
                  / s-Thu / s-Fri / s-Sat / s-Sun
     weekday      = l-Mon / l-Tue / l-Wed
                  / l-Thu / l-Fri / l-Sat / l-Sun
     month        = s-Jan / s-Feb / s-Mar / s-Apr
                  / s-May / s-Jun / s-Jul / s-Aug
                  / s-Sep / s-Oct / s-Nov / s-Dec

     GMT   = %x47.4D.54 ; "GMT", case-sensitive

     s-Mon = %x4D.6F.6E ; "Mon", case-sensitive
     s-Tue = %x54.75.65 ; "Tue", case-sensitive
     s-Wed = %x57.65.64 ; "Wed", case-sensitive
     s-Thu = %x54.68.75 ; "Thu", case-sensitive
     s-Fri = %x46.72.69 ; "Fri", case-sensitive
     s-Sat = %x53.61.74 ; "Sat", case-sensitive
     s-Sun = %x53.75.6E ; "Sun", case-sensitive

     l-Mon = %x4D.6F.6E.64.61.79          ; "Monday", case-sensitive
     l-Tue = %x54.       ; "Tuesday", case-sensitive
     l-Wed = %x57.65.64.6E. ; "Wednesday", case-sensitive
     l-Thu = %x54.    ; "Thursday", case-sensitive
     l-Fri = %x46.          ; "Friday", case-sensitive
     l-Sat = %x53.    ; "Saturday", case-sensitive
     l-Sun = %x53.75.6E.64.61.79          ; "Sunday", case-sensitive

     s-Jan = %x4A.61.6E ; "Jan", case-sensitive
     s-Feb = %x46.65.62 ; "Feb", case-sensitive
     s-Mar = %x4D.61.72 ; "Mar", case-sensitive
     s-Apr = %x41.70.72 ; "Apr", case-sensitive
     s-May = %x4D.61.79 ; "May", case-sensitive
     s-Jun = %x4A.75.6E ; "Jun", case-sensitive
     s-Jul = %x4A.75.6C ; "Jul", case-sensitive
     s-Aug = %x41.75.67 ; "Aug", case-sensitive
     s-Sep = %x53.65.70 ; "Sep", case-sensitive
     s-Oct = %x4F.63.74 ; "Oct", case-sensitive
     s-Nov = %x4E.6F.76 ; "Nov", case-sensitive
     s-Dec = %x44.65.63 ; "Dec", case-sensitive

   Note: HTTP requirements for the date/time stamp format apply only to
   their usage within the protocol stream.  Clients and servers are not
   required to use these formats for user presentation, request logging,

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3.4.  Transfer Codings

   Transfer-coding values are used to indicate an encoding
   transformation that has been, can be, or may need to be applied to an
   entity-body in order to ensure "safe transport" through the network.
   This differs from a content coding in that the transfer-coding is a
   property of the message, not of the original entity.

     transfer-coding         = "chunked" / transfer-extension
     transfer-extension      = token *( OWS ";" OWS parameter )

   Parameters are in the form of attribute/value pairs.

     parameter               = attribute BWS "=" BWS value
     attribute               = token
     value                   = token / quoted-string

   All transfer-coding values are case-insensitive.  HTTP/1.1 uses
   transfer-coding values in the TE header field (Section 8.5) and in
   the Transfer-Encoding header field (Section 8.7).

   Whenever a transfer-coding is applied to a message-body, the set of
   transfer-codings MUST include "chunked", unless the message indicates
   it is terminated by closing the connection.  When the "chunked"
   transfer-coding is used, it MUST be the last transfer-coding applied
   to the message-body.  The "chunked" transfer-coding MUST NOT be
   applied more than once to a message-body.  These rules allow the
   recipient to determine the transfer-length of the message
   (Section 4.4).

   Transfer-codings are analogous to the Content-Transfer-Encoding
   values of MIME [RFC2045], which were designed to enable safe
   transport of binary data over a 7-bit transport service.  However,
   safe transport has a different focus for an 8bit-clean transfer
   protocol.  In HTTP, the only unsafe characteristic of message-bodies
   is the difficulty in determining the exact body length (Section 4.4),
   or the desire to encrypt data over a shared transport.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   transfer-coding value tokens.  Initially, the registry contains the
   following tokens: "chunked" (Section 3.4.1), "gzip", "compress", and
   "deflate" (Section 3.2 of [Part3]).

   New transfer-coding value tokens SHOULD be registered in the same way
   as new content-coding value tokens (Section 3.2 of [Part3]).

   A server which receives an entity-body with a transfer-coding it does
   not understand SHOULD return 501 (Not Implemented), and close the

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   connection.  A server MUST NOT send transfer-codings to an HTTP/1.0

3.4.1.  Chunked Transfer Coding

   The chunked encoding modifies the body of a message in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing entity-header fields.
   This allows dynamically produced content to be transferred along with
   the information necessary for the recipient to verify that it has
   received the full message.

     Chunked-Body   = *chunk

     chunk          = chunk-size *WSP [ chunk-ext ] CRLF
                      chunk-data CRLF
     chunk-size     = 1*HEXDIG
     last-chunk     = 1*("0") *WSP [ chunk-ext ] CRLF

     chunk-ext      = *( ";" *WSP chunk-ext-name
                         [ "=" chunk-ext-val ] *WSP )
     chunk-ext-name = token
     chunk-ext-val  = token / quoted-string
     chunk-data     = 1*OCTET ; a sequence of chunk-size octets
     trailer-part   = *(entity-header CRLF)

   The chunk-size field is a string of hex digits indicating the size of
   the chunk-data in octets.  The chunked encoding is ended by any chunk
   whose size is zero, followed by the trailer, which is terminated by
   an empty line.

   The trailer allows the sender to include additional HTTP header
   fields at the end of the message.  The Trailer header field can be
   used to indicate which header fields are included in a trailer (see
   Section 8.6).

   A server using chunked transfer-coding in a response MUST NOT use the
   trailer for any header fields unless at least one of the following is

   1.  the request included a TE header field that indicates "trailers"
       is acceptable in the transfer-coding of the response, as
       described in Section 8.5; or,

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   2.  the server is the origin server for the response, the trailer
       fields consist entirely of optional metadata, and the recipient
       could use the message (in a manner acceptable to the origin
       server) without receiving this metadata.  In other words, the
       origin server is willing to accept the possibility that the
       trailer fields might be silently discarded along the path to the

   This requirement prevents an interoperability failure when the
   message is being received by an HTTP/1.1 (or later) proxy and
   forwarded to an HTTP/1.0 recipient.  It avoids a situation where
   compliance with the protocol would have necessitated a possibly
   infinite buffer on the proxy.

   A process for decoding the "chunked" transfer-coding can be
   represented in pseudo-code as:

     length := 0
     read chunk-size, chunk-ext (if any) and CRLF
     while (chunk-size > 0) {
        read chunk-data and CRLF
        append chunk-data to entity-body
        length := length + chunk-size
        read chunk-size and CRLF
     read entity-header
     while (entity-header not empty) {
        append entity-header to existing header fields
        read entity-header
     Content-Length := length
     Remove "chunked" from Transfer-Encoding

   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer-coding, and MUST ignore chunk-ext extensions they
   do not understand.

3.5.  Product Tokens

   Product tokens are used to allow communicating applications to
   identify themselves by software name and version.  Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by white space.  By
   convention, the products are listed in order of their significance
   for identifying the application.

     product         = token ["/" product-version]
     product-version = token

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       User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       Server: Apache/0.8.4

   Product tokens SHOULD be short and to the point.  They MUST NOT be
   used for advertising or other non-essential information.  Although
   any token character MAY appear in a product-version, this token
   SHOULD only be used for a version identifier (i.e., successive
   versions of the same product SHOULD only differ in the product-
   version portion of the product value).

4.  HTTP Message

4.1.  Message Types

   HTTP messages consist of requests from client to server and responses
   from server to client.

     HTTP-message   = Request / Response     ; HTTP/1.1 messages

   Request (Section 5) and Response (Section 6) messages use the generic
   message format of [RFC5322] for transferring entities (the payload of
   the message).  Both types of message consist of a start-line, zero or
   more header fields (also known as "headers"), an empty line (i.e., a
   line with nothing preceding the CRLF) indicating the end of the
   header fields, and possibly a message-body.

     generic-message = start-line
                       *(message-header CRLF)
                       [ message-body ]
     start-line      = Request-Line / Status-Line

   In the interest of robustness, servers SHOULD ignore any empty
   line(s) received where a Request-Line is expected.  In other words,
   if the server is reading the protocol stream at the beginning of a
   message and receives a CRLF first, it should ignore the CRLF.

   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
   after a POST request.  To restate what is explicitly forbidden by the
   BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an
   extra CRLF.

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4.2.  Message Headers

   HTTP header fields, which include general-header (Section 4.5),
   request-header (Section 4 of [Part2]), response-header (Section 6 of
   [Part2]), and entity-header (Section 4.1 of [Part3]) fields, follow
   the same generic format as that given in Section 2.1 of [RFC5322].
   Each header field consists of a name followed by a colon (":") and
   the field value.  Field names are case-insensitive.  The field value
   MAY be preceded by any amount of LWS, though a single SP is
   preferred.  Header fields can be extended over multiple lines by
   preceding each extra line with at least one SP or HTAB.  Applications
   ought to follow "common form", where one is known or indicated, when
   generating HTTP constructs, since there might exist some
   implementations that fail to accept anything beyond the common forms.

     message-header = field-name ":" [ field-value ]
     field-name     = token
     field-value    = *( field-content / OWS )
     field-content  = <field content>

   [[anchor1: whitespace between field-name and colon is an error and
   MUST NOT be accepted]]

   The field-content does not include any leading or trailing LWS:
   linear white space occurring before the first non-whitespace
   character of the field-value or after the last non-whitespace
   character of the field-value.  Such leading or trailing LWS MAY be
   removed without changing the semantics of the field value.  Any LWS
   that occurs between field-content MAY be replaced with a single SP
   before interpreting the field value or forwarding the message

   The order in which header fields with differing field names are
   received is not significant.  However, it is "good practice" to send
   general-header fields first, followed by request-header or response-
   header fields, and ending with the entity-header fields.

   Multiple message-header fields with the same field-name MAY be
   present in a message if and only if the entire field-value for that
   header field is defined as a comma-separated list [i.e., #(values)].
   It MUST be possible to combine the multiple header fields into one
   "field-name: field-value" pair, without changing the semantics of the
   message, by appending each subsequent field-value to the first, each
   separated by a comma.  The order in which header fields with the same
   field-name are received is therefore significant to the
   interpretation of the combined field value, and thus a proxy MUST NOT
   change the order of these field values when a message is forwarded.

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      Note: the "Set-Cookie" header as implemented in practice (as
      opposed to how it is specified in [RFC2109]) can occur multiple
      times, but does not use the list syntax, and thus cannot be
      combined into a single line.  (See Appendix A.2.3 of [Kri2001] for
      details.)  Also note that the Set-Cookie2 header specified in
      [RFC2965] does not share this problem.

4.3.  Message Body

   The message-body (if any) of an HTTP message is used to carry the
   entity-body associated with the request or response.  The message-
   body differs from the entity-body only when a transfer-coding has
   been applied, as indicated by the Transfer-Encoding header field
   (Section 8.7).

     message-body = entity-body
                  / <entity-body encoded as per Transfer-Encoding>

   Transfer-Encoding MUST be used to indicate any transfer-codings
   applied by an application to ensure safe and proper transfer of the
   message.  Transfer-Encoding is a property of the message, not of the
   entity, and thus MAY be added or removed by any application along the
   request/response chain.  (However, Section 3.4 places restrictions on
   when certain transfer-codings may be used.)

   The rules for when a message-body is allowed in a message differ for
   requests and responses.

   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's message-headers.  A message-body MUST NOT be included
   in a request if the specification of the request method (Section 3 of
   [Part2]) explicitly disallows an entity-body in requests.  When a
   request message contains both a message-body of non-zero length and a
   method that does not define any semantics for that request message-
   body, then an origin server SHOULD either ignore the message-body or
   respond with an appropriate error message (e.g., 413).  A proxy or
   gateway, when presented the same request, SHOULD either forward the
   request inbound with the message-body or ignore the message-body when
   determining a response.

   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (Section 6.1.1).  All responses to the HEAD request
   method MUST NOT include a message-body, even though the presence of
   entity-header fields might lead one to believe they do.  All 1xx
   (informational), 204 (No Content), and 304 (Not Modified) responses
   MUST NOT include a message-body.  All other responses do include a

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   message-body, although it MAY be of zero length.

4.4.  Message Length

   The transfer-length of a message is the length of the message-body as
   it appears in the message; that is, after any transfer-codings have
   been applied.  When a message-body is included with a message, the
   transfer-length of that body is determined by one of the following
   (in order of precedence):

   1.  Any response message which "MUST NOT" include a message-body
       (such as the 1xx, 204, and 304 responses and any response to a
       HEAD request) is always terminated by the first empty line after
       the header fields, regardless of the entity-header fields present
       in the message.

   2.  If a Transfer-Encoding header field (Section 8.7) is present and
       the "chunked" transfer-coding (Section 3.4) is used, the
       transfer-length is defined by the use of this transfer-coding.
       If a Transfer-Encoding header field is present and the "chunked"
       transfer-coding is not present, the transfer-length is defined by
       the sender closing the connection.

   3.  If a Content-Length header field (Section 8.2) is present, its
       decimal value in OCTETs represents both the entity-length and the
       transfer-length.  The Content-Length header field MUST NOT be
       sent if these two lengths are different (i.e., if a Transfer-
       Encoding header field is present).  If a message is received with
       both a Transfer-Encoding header field and a Content-Length header
       field, the latter MUST be ignored.

   4.  If the message uses the media type "multipart/byteranges", and
       the transfer-length is not otherwise specified, then this self-
       delimiting media type defines the transfer-length.  This media
       type MUST NOT be used unless the sender knows that the recipient
       can parse it; the presence in a request of a Range header with
       multiple byte-range specifiers from a 1.1 client implies that the
       client can parse multipart/byteranges responses.

          A range header might be forwarded by a 1.0 proxy that does not
          understand multipart/byteranges; in this case the server MUST
          delimit the message using methods defined in items 1, 3 or 5
          of this section.

   5.  By the server closing the connection.  (Closing the connection
       cannot be used to indicate the end of a request body, since that
       would leave no possibility for the server to send back a

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   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
   containing a message-body MUST include a valid Content-Length header
   field unless the server is known to be HTTP/1.1 compliant.  If a
   request contains a message-body and a Content-Length is not given,
   the server SHOULD respond with 400 (Bad Request) if it cannot
   determine the length of the message, or with 411 (Length Required) if
   it wishes to insist on receiving a valid Content-Length.

   All HTTP/1.1 applications that receive entities MUST accept the
   "chunked" transfer-coding (Section 3.4), thus allowing this mechanism
   to be used for messages when the message length cannot be determined
   in advance.

   Messages MUST NOT include both a Content-Length header field and a
   transfer-coding.  If the message does include a transfer-coding, the
   Content-Length MUST be ignored.

   When a Content-Length is given in a message where a message-body is
   allowed, its field value MUST exactly match the number of OCTETs in
   the message-body.  HTTP/1.1 user agents MUST notify the user when an
   invalid length is received and detected.

4.5.  General Header Fields

   There are a few header fields which have general applicability for
   both request and response messages, but which do not apply to the
   entity being transferred.  These header fields apply only to the
   message being transmitted.

     general-header = Cache-Control            ; [Part6], Section 16.2
                    / Connection               ; Section 8.1
                    / Date                     ; Section 8.3
                    / Pragma                   ; [Part6], Section 16.4
                    / Trailer                  ; Section 8.6
                    / Transfer-Encoding        ; Section 8.7
                    / Upgrade                  ; Section 8.8
                    / Via                      ; Section 8.9
                    / Warning                  ; [Part6], Section 16.6

   General-header field names can be extended reliably only in
   combination with a change in the protocol version.  However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general-header fields.  Unrecognized header fields are treated as
   entity-header fields.

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5.  Request

   A request message from a client to a server includes, within the
   first line of that message, the method to be applied to the resource,
   the identifier of the resource, and the protocol version in use.

     Request       = Request-Line              ; Section 5.1
                     *(( general-header        ; Section 4.5
                      / request-header         ; [Part2], Section 4
                      / entity-header ) CRLF)  ; [Part3], Section 4.1
                     [ message-body ]          ; Section 4.3

5.1.  Request-Line

   The Request-Line begins with a method token, followed by the Request-
   URI and the protocol version, and ending with CRLF.  The elements are
   separated by SP characters.  No CR or LF is allowed except in the
   final CRLF sequence.

     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF

5.1.1.  Method

   The Method token indicates the method to be performed on the resource
   identified by the Request-URI.  The method is case-sensitive.

     Method         = token

5.1.2.  Request-URI

   The Request-URI is a Uniform Resource Identifier (Section 3.2) and
   identifies the resource upon which to apply the request.

     Request-URI    = "*"
                    / absolute-URI
                    / ( path-absolute [ "?" query ] )
                    / authority

   The four options for Request-URI are dependent on the nature of the
   request.  The asterisk "*" means that the request does not apply to a
   particular resource, but to the server itself, and is only allowed
   when the method used does not necessarily apply to a resource.  One
   example would be

       OPTIONS * HTTP/1.1

   The absolute-URI form is REQUIRED when the request is being made to a

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   proxy.  The proxy is requested to forward the request or service it
   from a valid cache, and return the response.  Note that the proxy MAY
   forward the request on to another proxy or directly to the server
   specified by the absolute-URI.  In order to avoid request loops, a
   proxy MUST be able to recognize all of its server names, including
   any aliases, local variations, and the numeric IP address.  An
   example Request-Line would be:

       GET HTTP/1.1

   To allow for transition to absolute-URIs in all requests in future
   versions of HTTP, all HTTP/1.1 servers MUST accept the absolute-URI
   form in requests, even though HTTP/1.1 clients will only generate
   them in requests to proxies.

   The authority form is only used by the CONNECT method (Section 8.9 of

   The most common form of Request-URI is that used to identify a
   resource on an origin server or gateway.  In this case the absolute
   path of the URI MUST be transmitted (see Section 3.2.1, path-
   absolute) as the Request-URI, and the network location of the URI
   (authority) MUST be transmitted in a Host header field.  For example,
   a client wishing to retrieve the resource above directly from the
   origin server would create a TCP connection to port 80 of the host
   "" and send the lines:

       GET /pub/WWW/TheProject.html HTTP/1.1

   followed by the remainder of the Request.  Note that the absolute
   path cannot be empty; if none is present in the original URI, it MUST
   be given as "/" (the server root).

   The Request-URI is transmitted in the format specified in
   Section 3.2.1.  If the Request-URI is encoded using the "% HEXDIG
   HEXDIG" encoding ([RFC3986], Section 2.4), the origin server MUST
   decode the Request-URI in order to properly interpret the request.
   Servers SHOULD respond to invalid Request-URIs with an appropriate
   status code.

   A transparent proxy MUST NOT rewrite the "path-absolute" part of the
   received Request-URI when forwarding it to the next inbound server,
   except as noted above to replace a null path-absolute with "/".

      Note: The "no rewrite" rule prevents the proxy from changing the
      meaning of the request when the origin server is improperly using
      a non-reserved URI character for a reserved purpose.  Implementors

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      should be aware that some pre-HTTP/1.1 proxies have been known to
      rewrite the Request-URI.

5.2.  The Resource Identified by a Request

   The exact resource identified by an Internet request is determined by
   examining both the Request-URI and the Host header field.

   An origin server that does not allow resources to differ by the
   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request.  (But see
   Appendix C.1.1 for other requirements on Host support in HTTP/1.1.)

   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity host
   names) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:

   1.  If Request-URI is an absolute-URI, the host is part of the
       Request-URI.  Any Host header field value in the request MUST be

   2.  If the Request-URI is not an absolute-URI, and the request
       includes a Host header field, the host is determined by the Host
       header field value.

   3.  If the host as determined by rule 1 or 2 is not a valid host on
       the server, the response MUST be a 400 (Bad Request) error

   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.

6.  Response

   After receiving and interpreting a request message, a server responds
   with an HTTP response message.

     Response      = Status-Line               ; Section 6.1
                     *(( general-header        ; Section 4.5
                      / response-header        ; [Part2], Section 6
                      / entity-header ) CRLF)  ; [Part3], Section 4.1
                     [ message-body ]          ; Section 4.3

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6.1.  Status-Line

   The first line of a Response message is the Status-Line, consisting
   of the protocol version followed by a numeric status code and its
   associated textual phrase, with each element separated by SP
   characters.  No CR or LF is allowed except in the final CRLF

     Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

6.1.1.  Status Code and Reason Phrase

   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request.  These codes are fully
   defined in Section 9 of [Part2].  The Reason Phrase exists for the
   sole purpose of providing a textual description associated with the
   numeric status code, out of deference to earlier Internet application
   protocols that were more frequently used with interactive text
   clients.  A client SHOULD ignore the content of the Reason Phrase.

   The first digit of the Status-Code defines the class of response.
   The last two digits do not have any categorization role.  There are 5
   values for the first digit:

   o  1xx: Informational - Request received, continuing process

   o  2xx: Success - The action was successfully received, understood,
      and accepted

   o  3xx: Redirection - Further action must be taken in order to
      complete the request

   o  4xx: Client Error - The request contains bad syntax or cannot be

   o  5xx: Server Error - The server failed to fulfill an apparently
      valid request

     Status-Code    = 3DIGIT
     Reason-Phrase  = *<TEXT, excluding CR, LF>

7.  Connections

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7.1.  Persistent Connections

7.1.1.  Purpose

   Prior to persistent connections, a separate TCP connection was
   established to fetch each URL, increasing the load on HTTP servers
   and causing congestion on the Internet.  The use of inline images and
   other associated data often require a client to make multiple
   requests of the same server in a short amount of time.  Analysis of
   these performance problems and results from a prototype
   implementation are available [Pad1995] [Spe].  Implementation
   experience and measurements of actual HTTP/1.1 (RFC 2068)
   implementations show good results [Nie1997].  Alternatives have also
   been explored, for example, T/TCP [Tou1998].

   Persistent HTTP connections have a number of advantages:

   o  By opening and closing fewer TCP connections, CPU time is saved in
      routers and hosts (clients, servers, proxies, gateways, tunnels,
      or caches), and memory used for TCP protocol control blocks can be
      saved in hosts.

   o  HTTP requests and responses can be pipelined on a connection.
      Pipelining allows a client to make multiple requests without
      waiting for each response, allowing a single TCP connection to be
      used much more efficiently, with much lower elapsed time.

   o  Network congestion is reduced by reducing the number of packets
      caused by TCP opens, and by allowing TCP sufficient time to
      determine the congestion state of the network.

   o  Latency on subsequent requests is reduced since there is no time
      spent in TCP's connection opening handshake.

   o  HTTP can evolve more gracefully, since errors can be reported
      without the penalty of closing the TCP connection.  Clients using
      future versions of HTTP might optimistically try a new feature,
      but if communicating with an older server, retry with old
      semantics after an error is reported.

   HTTP implementations SHOULD implement persistent connections.

7.1.2.  Overall Operation

   A significant difference between HTTP/1.1 and earlier versions of
   HTTP is that persistent connections are the default behavior of any
   HTTP connection.  That is, unless otherwise indicated, the client
   SHOULD assume that the server will maintain a persistent connection,

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   even after error responses from the server.

   Persistent connections provide a mechanism by which a client and a
   server can signal the close of a TCP connection.  This signaling
   takes place using the Connection header field (Section 8.1).  Once a
   close has been signaled, the client MUST NOT send any more requests
   on that connection.  Negotiation

   An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
   maintain a persistent connection unless a Connection header including
   the connection-token "close" was sent in the request.  If the server
   chooses to close the connection immediately after sending the
   response, it SHOULD send a Connection header including the
   connection-token close.

   An HTTP/1.1 client MAY expect a connection to remain open, but would
   decide to keep it open based on whether the response from a server
   contains a Connection header with the connection-token close.  In
   case the client does not want to maintain a connection for more than
   that request, it SHOULD send a Connection header including the
   connection-token close.

   If either the client or the server sends the close token in the
   Connection header, that request becomes the last one for the

   Clients and servers SHOULD NOT assume that a persistent connection is
   maintained for HTTP versions less than 1.1 unless it is explicitly
   signaled.  See Appendix C.2 for more information on backward
   compatibility with HTTP/1.0 clients.

   In order to remain persistent, all messages on the connection MUST
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in Section 4.4.  Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response).  A server MUST send its responses to those requests in the
   same order that the requests were received.

   Clients which assume persistent connections and pipeline immediately
   after connection establishment SHOULD be prepared to retry their
   connection if the first pipelined attempt fails.  If a client does
   such a retry, it MUST NOT pipeline before it knows the connection is

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   persistent.  Clients MUST also be prepared to resend their requests
   if the server closes the connection before sending all of the
   corresponding responses.

   Clients SHOULD NOT pipeline requests using non-idempotent methods or
   non-idempotent sequences of methods (see Section 8.1.2 of [Part2]).
   Otherwise, a premature termination of the transport connection could
   lead to indeterminate results.  A client wishing to send a non-
   idempotent request SHOULD wait to send that request until it has
   received the response status for the previous request.

7.1.3.  Proxy Servers

   It is especially important that proxies correctly implement the
   properties of the Connection header field as specified in
   Section 8.1.

   The proxy server MUST signal persistent connections separately with
   its clients and the origin servers (or other proxy servers) that it
   connects to.  Each persistent connection applies to only one
   transport link.

   A proxy server MUST NOT establish a HTTP/1.1 persistent connection
   with an HTTP/1.0 client (but see [RFC2068] for information and
   discussion of the problems with the Keep-Alive header implemented by
   many HTTP/1.0 clients).

7.1.4.  Practical Considerations

   Servers will usually have some time-out value beyond which they will
   no longer maintain an inactive connection.  Proxy servers might make
   this a higher value since it is likely that the client will be making
   more connections through the same server.  The use of persistent
   connections places no requirements on the length (or existence) of
   this time-out for either the client or the server.

   When a client or server wishes to time-out it SHOULD issue a graceful
   close on the transport connection.  Clients and servers SHOULD both
   constantly watch for the other side of the transport close, and
   respond to it as appropriate.  If a client or server does not detect
   the other side's close promptly it could cause unnecessary resource
   drain on the network.

   A client, server, or proxy MAY close the transport connection at any
   time.  For example, a client might have started to send a new request
   at the same time that the server has decided to close the "idle"
   connection.  From the server's point of view, the connection is being
   closed while it was idle, but from the client's point of view, a

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   request is in progress.

   This means that clients, servers, and proxies MUST be able to recover
   from asynchronous close events.  Client software SHOULD reopen the
   transport connection and retransmit the aborted sequence of requests
   without user interaction so long as the request sequence is
   idempotent (see Section 8.1.2 of [Part2]).  Non-idempotent methods or
   sequences MUST NOT be automatically retried, although user agents MAY
   offer a human operator the choice of retrying the request(s).
   Confirmation by user-agent software with semantic understanding of
   the application MAY substitute for user confirmation.  The automatic
   retry SHOULD NOT be repeated if the second sequence of requests

   Servers SHOULD always respond to at least one request per connection,
   if at all possible.  Servers SHOULD NOT close a connection in the
   middle of transmitting a response, unless a network or client failure
   is suspected.

   Clients that use persistent connections SHOULD limit the number of
   simultaneous connections that they maintain to a given server.  A
   single-user client SHOULD NOT maintain more than 2 connections with
   any server or proxy.  A proxy SHOULD use up to 2*N connections to
   another server or proxy, where N is the number of simultaneously
   active users.  These guidelines are intended to improve HTTP response
   times and avoid congestion.

7.2.  Message Transmission Requirements

7.2.1.  Persistent Connections and Flow Control

   HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
   flow control mechanisms to resolve temporary overloads, rather than
   terminating connections with the expectation that clients will retry.
   The latter technique can exacerbate network congestion.

7.2.2.  Monitoring Connections for Error Status Messages

   An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
   the network connection for an error status while it is transmitting
   the request.  If the client sees an error status, it SHOULD
   immediately cease transmitting the body.  If the body is being sent
   using a "chunked" encoding (Section 3.4), a zero length chunk and
   empty trailer MAY be used to prematurely mark the end of the message.
   If the body was preceded by a Content-Length header, the client MUST
   close the connection.

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7.2.3.  Use of the 100 (Continue) Status

   The purpose of the 100 (Continue) status (see Section 9.1.1 of
   [Part2]) is to allow a client that is sending a request message with
   a request body to determine if the origin server is willing to accept
   the request (based on the request headers) before the client sends
   the request body.  In some cases, it might either be inappropriate or
   highly inefficient for the client to send the body if the server will
   reject the message without looking at the body.

   Requirements for HTTP/1.1 clients:

   o  If a client will wait for a 100 (Continue) response before sending
      the request body, it MUST send an Expect request-header field
      (Section 10.2 of [Part2]) with the "100-continue" expectation.

   o  A client MUST NOT send an Expect request-header field (Section
      10.2 of [Part2]) with the "100-continue" expectation if it does
      not intend to send a request body.

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client may send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) status
   or a 100 (Continue) status.  Therefore, when a client sends this
   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status, the client SHOULD NOT wait
   for an indefinite period before sending the request body.

   Requirements for HTTP/1.1 origin servers:

   o  Upon receiving a request which includes an Expect request-header
      field with the "100-continue" expectation, an origin server MUST
      either respond with 100 (Continue) status and continue to read
      from the input stream, or respond with a final status code.  The
      origin server MUST NOT wait for the request body before sending
      the 100 (Continue) response.  If it responds with a final status
      code, it MAY close the transport connection or it MAY continue to
      read and discard the rest of the request.  It MUST NOT perform the
      requested method if it returns a final status code.

   o  An origin server SHOULD NOT send a 100 (Continue) response if the
      request message does not include an Expect request-header field
      with the "100-continue" expectation, and MUST NOT send a 100
      (Continue) response if such a request comes from an HTTP/1.0 (or
      earlier) client.  There is an exception to this rule: for
      compatibility with [RFC2068], a server MAY send a 100 (Continue)
      status in response to an HTTP/1.1 PUT or POST request that does
      not include an Expect request-header field with the "100-continue"

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      expectation.  This exception, the purpose of which is to minimize
      any client processing delays associated with an undeclared wait
      for 100 (Continue) status, applies only to HTTP/1.1 requests, and
      not to requests with any other HTTP-version value.

   o  An origin server MAY omit a 100 (Continue) response if it has
      already received some or all of the request body for the
      corresponding request.

   o  An origin server that sends a 100 (Continue) response MUST
      ultimately send a final status code, once the request body is
      received and processed, unless it terminates the transport
      connection prematurely.

   o  If an origin server receives a request that does not include an
      Expect request-header field with the "100-continue" expectation,
      the request includes a request body, and the server responds with
      a final status code before reading the entire request body from
      the transport connection, then the server SHOULD NOT close the
      transport connection until it has read the entire request, or
      until the client closes the connection.  Otherwise, the client
      might not reliably receive the response message.  However, this
      requirement is not be construed as preventing a server from
      defending itself against denial-of-service attacks, or from badly
      broken client implementations.

   Requirements for HTTP/1.1 proxies:

   o  If a proxy receives a request that includes an Expect request-
      header field with the "100-continue" expectation, and the proxy
      either knows that the next-hop server complies with HTTP/1.1 or
      higher, or does not know the HTTP version of the next-hop server,
      it MUST forward the request, including the Expect header field.

   o  If the proxy knows that the version of the next-hop server is
      HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
      respond with a 417 (Expectation Failed) status.

   o  Proxies SHOULD maintain a cache recording the HTTP version numbers
      received from recently-referenced next-hop servers.

   o  A proxy MUST NOT forward a 100 (Continue) response if the request
      message was received from an HTTP/1.0 (or earlier) client and did
      not include an Expect request-header field with the "100-continue"
      expectation.  This requirement overrides the general rule for
      forwarding of 1xx responses (see Section 9.1 of [Part2]).

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7.2.4.  Client Behavior if Server Prematurely Closes Connection

   If an HTTP/1.1 client sends a request which includes a request body,
   but which does not include an Expect request-header field with the
   "100-continue" expectation, and if the client is not directly
   connected to an HTTP/1.1 origin server, and if the client sees the
   connection close before receiving any status from the server, the
   client SHOULD retry the request.  If the client does retry this
   request, it MAY use the following "binary exponential backoff"
   algorithm to be assured of obtaining a reliable response:

   1.  Initiate a new connection to the server

   2.  Transmit the request-headers

   3.  Initialize a variable R to the estimated round-trip time to the
       server (e.g., based on the time it took to establish the
       connection), or to a constant value of 5 seconds if the round-
       trip time is not available.

   4.  Compute T = R * (2**N), where N is the number of previous retries
       of this request.

   5.  Wait either for an error response from the server, or for T
       seconds (whichever comes first)

   6.  If no error response is received, after T seconds transmit the
       body of the request.

   7.  If client sees that the connection is closed prematurely, repeat
       from step 1 until the request is accepted, an error response is
       received, or the user becomes impatient and terminates the retry

   If at any point an error status is received, the client

   o  SHOULD NOT continue and

   o  SHOULD close the connection if it has not completed sending the
      request message.

8.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to message framing and transport protocols.

   For entity-header fields, both sender and recipient refer to either

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   the client or the server, depending on who sends and who receives the

8.1.  Connection

   The general-header field "Connection" allows the sender to specify
   options that are desired for that particular connection and MUST NOT
   be communicated by proxies over further connections.

   The Connection header's value has the following grammar:

     Connection       = "Connection" ":" OWS Connection-v
     Connection-v     = 1#connection-token
     connection-token = token

   HTTP/1.1 proxies MUST parse the Connection header field before a
   message is forwarded and, for each connection-token in this field,
   remove any header field(s) from the message with the same name as the
   connection-token.  Connection options are signaled by the presence of
   a connection-token in the Connection header field, not by any
   corresponding additional header field(s), since the additional header
   field may not be sent if there are no parameters associated with that
   connection option.

   Message headers listed in the Connection header MUST NOT include end-
   to-end headers, such as Cache-Control.

   HTTP/1.1 defines the "close" connection option for the sender to
   signal that the connection will be closed after completion of the
   response.  For example,

     Connection: close

   in either the request or the response header fields indicates that
   the connection SHOULD NOT be considered `persistent' (Section 7.1)
   after the current request/response is complete.

   An HTTP/1.1 client that does not support persistent connections MUST
   include the "close" connection option in every request message.

   An HTTP/1.1 server that does not support persistent connections MUST
   include the "close" connection option in every response message that
   does not have a 1xx (informational) status code.

   A system receiving an HTTP/1.0 (or lower-version) message that
   includes a Connection header MUST, for each connection-token in this
   field, remove and ignore any header field(s) from the message with
   the same name as the connection-token.  This protects against

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   mistaken forwarding of such header fields by pre-HTTP/1.1 proxies.
   See Appendix C.2.

8.2.  Content-Length

   The entity-header field "Content-Length" indicates the size of the
   entity-body, in decimal number of OCTETs, sent to the recipient or,
   in the case of the HEAD method, the size of the entity-body that
   would have been sent had the request been a GET.

     Content-Length   = "Content-Length" ":" OWS 1*Content-Length-v
     Content-Length-v = 1*DIGIT

   An example is

     Content-Length: 3495

   Applications SHOULD use this field to indicate the transfer-length of
   the message-body, unless this is prohibited by the rules in
   Section 4.4.

   Any Content-Length greater than or equal to zero is a valid value.
   Section 4.4 describes how to determine the length of a message-body
   if a Content-Length is not given.

   Note that the meaning of this field is significantly different from
   the corresponding definition in MIME, where it is an optional field
   used within the "message/external-body" content-type.  In HTTP, it
   SHOULD be sent whenever the message's length can be determined prior
   to being transferred, unless this is prohibited by the rules in
   Section 4.4.

8.3.  Date

   The general-header field "Date" represents the date and time at which
   the message was originated, having the same semantics as orig-date in
   Section 3.6.1 of [RFC5322].  The field value is an HTTP-date, as
   described in Section 3.3.1; it MUST be sent in rfc1123-date format.

     Date   = "Date" ":" OWS Date-v
     Date-v = HTTP-date

   An example is

     Date: Tue, 15 Nov 1994 08:12:31 GMT

   Origin servers MUST include a Date header field in all responses,
   except in these cases:

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   1.  If the response status code is 100 (Continue) or 101 (Switching
       Protocols), the response MAY include a Date header field, at the
       server's option.

   2.  If the response status code conveys a server error, e.g. 500
       (Internal Server Error) or 503 (Service Unavailable), and it is
       inconvenient or impossible to generate a valid Date.

   3.  If the server does not have a clock that can provide a reasonable
       approximation of the current time, its responses MUST NOT include
       a Date header field.  In this case, the rules in Section 8.3.1
       MUST be followed.

   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient or gatewayed via a protocol which requires a Date.  An HTTP
   implementation without a clock MUST NOT cache responses without
   revalidating them on every use.  An HTTP cache, especially a shared
   cache, SHOULD use a mechanism, such as NTP [RFC1305], to synchronize
   its clock with a reliable external standard.

   Clients SHOULD only send a Date header field in messages that include
   an entity-body, as in the case of the PUT and POST requests, and even
   then it is optional.  A client without a clock MUST NOT send a Date
   header field in a request.

   The HTTP-date sent in a Date header SHOULD NOT represent a date and
   time subsequent to the generation of the message.  It SHOULD
   represent the best available approximation of the date and time of
   message generation, unless the implementation has no means of
   generating a reasonably accurate date and time.  In theory, the date
   ought to represent the moment just before the entity is generated.
   In practice, the date can be generated at any time during the message
   origination without affecting its semantic value.

8.3.1.  Clockless Origin Server Operation

   Some origin server implementations might not have a clock available.
   An origin server without a clock MUST NOT assign Expires or Last-
   Modified values to a response, unless these values were associated
   with the resource by a system or user with a reliable clock.  It MAY
   assign an Expires value that is known, at or before server
   configuration time, to be in the past (this allows "pre-expiration"
   of responses without storing separate Expires values for each

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8.4.  Host

   The request-header field "Host" specifies the Internet host and port
   number of the resource being requested, as obtained from the original
   URI given by the user or referring resource (generally an HTTP URL,
   as described in Section 3.2.1).  The Host field value MUST represent
   the naming authority of the origin server or gateway given by the
   original URL.  This allows the origin server or gateway to
   differentiate between internally-ambiguous URLs, such as the root "/"
   URL of a server for multiple host names on a single IP address.

     Host   = "Host" ":" OWS Host-v
     Host-v = uri-host [ ":" port ] ; Section 3.2.1

   A "host" without any trailing port information implies the default
   port for the service requested (e.g., "80" for an HTTP URL).  For
   example, a request on the origin server for
   <> would properly include:

     GET /pub/WWW/ HTTP/1.1

   A client MUST include a Host header field in all HTTP/1.1 request
   messages.  If the requested URI does not include an Internet host
   name for the service being requested, then the Host header field MUST
   be given with an empty value.  An HTTP/1.1 proxy MUST ensure that any
   request message it forwards does contain an appropriate Host header
   field that identifies the service being requested by the proxy.  All
   Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
   status code to any HTTP/1.1 request message which lacks a Host header

   See Sections 5.2 and C.1.1 for other requirements relating to Host.

8.5.  TE

   The request-header field "TE" indicates what extension transfer-
   codings it is willing to accept in the response and whether or not it
   is willing to accept trailer fields in a chunked transfer-coding.
   Its value may consist of the keyword "trailers" and/or a comma-
   separated list of extension transfer-coding names with optional
   accept parameters (as described in Section 3.4).

     TE        = "TE" ":" OWS TE-v
     TE-v      = #t-codings
     t-codings = "trailers" / ( transfer-extension [ accept-params ] )

   The presence of the keyword "trailers" indicates that the client is

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   willing to accept trailer fields in a chunked transfer-coding, as
   defined in Section 3.4.1.  This keyword is reserved for use with
   transfer-coding values even though it does not itself represent a

   Examples of its use are:

     TE: deflate
     TE: trailers, deflate;q=0.5

   The TE header field only applies to the immediate connection.
   Therefore, the keyword MUST be supplied within a Connection header
   field (Section 8.1) whenever TE is present in an HTTP/1.1 message.

   A server tests whether a transfer-coding is acceptable, according to
   a TE field, using these rules:

   1.  The "chunked" transfer-coding is always acceptable.  If the
       keyword "trailers" is listed, the client indicates that it is
       willing to accept trailer fields in the chunked response on
       behalf of itself and any downstream clients.  The implication is
       that, if given, the client is stating that either all downstream
       clients are willing to accept trailer fields in the forwarded
       response, or that it will attempt to buffer the response on
       behalf of downstream recipients.

       Note: HTTP/1.1 does not define any means to limit the size of a
       chunked response such that a client can be assured of buffering
       the entire response.

   2.  If the transfer-coding being tested is one of the transfer-
       codings listed in the TE field, then it is acceptable unless it
       is accompanied by a qvalue of 0.  (As defined in Section 3.4 of
       [Part3], a qvalue of 0 means "not acceptable.")

   3.  If multiple transfer-codings are acceptable, then the acceptable
       transfer-coding with the highest non-zero qvalue is preferred.
       The "chunked" transfer-coding always has a qvalue of 1.

   If the TE field-value is empty or if no TE field is present, the only
   transfer-coding is "chunked".  A message with no transfer-coding is
   always acceptable.

8.6.  Trailer

   The general field "Trailer" indicates that the given set of header
   fields is present in the trailer of a message encoded with chunked

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     Trailer   = "Trailer" ":" OWS Trailer-v
     Trailer-v = 1#field-name

   An HTTP/1.1 message SHOULD include a Trailer header field in a
   message using chunked transfer-coding with a non-empty trailer.
   Doing so allows the recipient to know which header fields to expect
   in the trailer.

   If no Trailer header field is present, the trailer SHOULD NOT include
   any header fields.  See Section 3.4.1 for restrictions on the use of
   trailer fields in a "chunked" transfer-coding.

   Message header fields listed in the Trailer header field MUST NOT
   include the following header fields:

   o  Transfer-Encoding

   o  Content-Length

   o  Trailer

8.7.  Transfer-Encoding

   The general-header "Transfer-Encoding" field indicates what (if any)
   type of transformation has been applied to the message body in order
   to safely transfer it between the sender and the recipient.  This
   differs from the content-coding in that the transfer-coding is a
   property of the message, not of the entity.

     Transfer-Encoding   = "Transfer-Encoding" ":" OWS
     Transfer-Encoding-v = 1#transfer-coding

   Transfer-codings are defined in Section 3.4.  An example is:

     Transfer-Encoding: chunked

   If multiple encodings have been applied to an entity, the transfer-
   codings MUST be listed in the order in which they were applied.
   Additional information about the encoding parameters MAY be provided
   by other entity-header fields not defined by this specification.

   Many older HTTP/1.0 applications do not understand the Transfer-
   Encoding header.

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8.8.  Upgrade

   The general-header "Upgrade" allows the client to specify what
   additional communication protocols it supports and would like to use
   if the server finds it appropriate to switch protocols.  The server
   MUST use the Upgrade header field within a 101 (Switching Protocols)
   response to indicate which protocol(s) are being switched.

     Upgrade   = "Upgrade" ":" OWS Upgrade-v
     Upgrade-v = 1#product

   For example,

     Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

   The Upgrade header field is intended to provide a simple mechanism
   for transition from HTTP/1.1 to some other, incompatible protocol.
   It does so by allowing the client to advertise its desire to use
   another protocol, such as a later version of HTTP with a higher major
   version number, even though the current request has been made using
   HTTP/1.1.  This eases the difficult transition between incompatible
   protocols by allowing the client to initiate a request in the more
   commonly supported protocol while indicating to the server that it
   would like to use a "better" protocol if available (where "better" is
   determined by the server, possibly according to the nature of the
   method and/or resource being requested).

   The Upgrade header field only applies to switching application-layer
   protocols upon the existing transport-layer connection.  Upgrade
   cannot be used to insist on a protocol change; its acceptance and use
   by the server is optional.  The capabilities and nature of the
   application-layer communication after the protocol change is entirely
   dependent upon the new protocol chosen, although the first action
   after changing the protocol MUST be a response to the initial HTTP
   request containing the Upgrade header field.

   The Upgrade header field only applies to the immediate connection.
   Therefore, the upgrade keyword MUST be supplied within a Connection
   header field (Section 8.1) whenever Upgrade is present in an HTTP/1.1

   The Upgrade header field cannot be used to indicate a switch to a
   protocol on a different connection.  For that purpose, it is more
   appropriate to use a 301, 302, 303, or 305 redirection response.

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of Section 3.1 and future updates to this

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   specification.  Any token can be used as a protocol name; however, it
   will only be useful if both the client and server associate the name
   with the same protocol.

8.9.  Via

   The general-header field "Via" MUST be used by gateways and proxies
   to indicate the intermediate protocols and recipients between the
   user agent and the server on requests, and between the origin server
   and the client on responses.  It is analogous to the "Received" field
   defined in Section 3.6.7 of [RFC5322] and is intended to be used for
   tracking message forwards, avoiding request loops, and identifying
   the protocol capabilities of all senders along the request/response

     Via               = "Via" ":" OWS Via-v
     Via-v             = 1#( received-protocol RWS received-by
                             [ RWS comment ] )
     received-protocol = [ protocol-name "/" ] protocol-version
     protocol-name     = token
     protocol-version  = token
     received-by       = ( uri-host [ ":" port ] ) / pseudonym
     pseudonym         = token

   The received-protocol indicates the protocol version of the message
   received by the server or client along each segment of the request/
   response chain.  The received-protocol version is appended to the Via
   field value when the message is forwarded so that information about
   the protocol capabilities of upstream applications remains visible to
   all recipients.

   The protocol-name is optional if and only if it would be "HTTP".  The
   received-by field is normally the host and optional port number of a
   recipient server or client that subsequently forwarded the message.
   However, if the real host is considered to be sensitive information,
   it MAY be replaced by a pseudonym.  If the port is not given, it MAY
   be assumed to be the default port of the received-protocol.

   Multiple Via field values represents each proxy or gateway that has
   forwarded the message.  Each recipient MUST append its information
   such that the end result is ordered according to the sequence of
   forwarding applications.

   Comments MAY be used in the Via header field to identify the software
   of the recipient proxy or gateway, analogous to the User-Agent and
   Server header fields.  However, all comments in the Via field are
   optional and MAY be removed by any recipient prior to forwarding the

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   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at, which
   completes the request by forwarding it to the origin server at  The request received by would then
   have the following Via header field:

     Via: 1.0 fred, 1.1 (Apache/1.1)

   Proxies and gateways used as a portal through a network firewall
   SHOULD NOT, by default, forward the names and ports of hosts within
   the firewall region.  This information SHOULD only be propagated if
   explicitly enabled.  If not enabled, the received-by host of any host
   behind the firewall SHOULD be replaced by an appropriate pseudonym
   for that host.

   For organizations that have strong privacy requirements for hiding
   internal structures, a proxy MAY combine an ordered subsequence of
   Via header field entries with identical received-protocol values into
   a single such entry.  For example,

     Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

     Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   Applications SHOULD NOT combine multiple entries unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms.  Applications MUST NOT combine entries which
   have different received-protocol values.

9.  IANA Considerations

9.1.  Message Header Registration

   The Message Header Registry located at <
   assignments/message-headers/message-header-index.html> should be
   updated with the permanent registrations below (see [RFC3864]):

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   | Header Field Name | Protocol | Status   | Reference   |
   | Connection        | http     | standard | Section 8.1 |
   | Content-Length    | http     | standard | Section 8.2 |
   | Date              | http     | standard | Section 8.3 |
   | Host              | http     | standard | Section 8.4 |
   | TE                | http     | standard | Section 8.5 |
   | Trailer           | http     | standard | Section 8.6 |
   | Transfer-Encoding | http     | standard | Section 8.7 |
   | Upgrade           | http     | standard | Section 8.8 |
   | Via               | http     | standard | Section 8.9 |

   The change controller is: "IETF ( - Internet
   Engineering Task Force".

9.2.  URI Scheme Registration

   The entry for the "http" URI Scheme in the registry located at
   <> should be updated
   to point to Section 3.2.1 of this document (see [RFC4395]).

9.3.  Internet Media Type Registrations

   This document serves as the specification for the Internet media
   types "message/http" and "application/http".  The following is to be
   registered with IANA (see [RFC4288]).

9.3.1.  Internet Media Type message/http

   The message/http type can be used to enclose a single HTTP request or
   response message, provided that it obeys the MIME restrictions for
   all "message" types regarding line length and encodings.

   Type name:  message

   Subtype name:  http

   Required parameters:  none

   Optional parameters:  version, msgtype

      version:  The HTTP-Version number of the enclosed message (e.g.,
         "1.1").  If not present, the version can be determined from the
         first line of the body.

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      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the

   Encoding considerations:  only "7bit", "8bit", or "binary" are

   Security considerations:  none

   Interoperability considerations:  none

   Published specification:  This specification (see Section 9.3.1).

   Applications that use this media type:

   Additional information:

      Magic number(s):  none

      File extension(s):  none

      Macintosh file type code(s):  none

   Person and email address to contact for further information:  See
      Authors Section.

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author/Change controller:  IESG

9.3.2.  Internet Media Type application/http

   The application/http type can be used to enclose a pipeline of one or
   more HTTP request or response messages (not intermixed).

   Type name:  application

   Subtype name:  http

   Required parameters:  none

   Optional parameters:  version, msgtype

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      version:  The HTTP-Version number of the enclosed messages (e.g.,
         "1.1").  If not present, the version can be determined from the
         first line of the body.

      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the

   Encoding considerations:  HTTP messages enclosed by this type are in
      "binary" format; use of an appropriate Content-Transfer-Encoding
      is required when transmitted via E-mail.

   Security considerations:  none

   Interoperability considerations:  none

   Published specification:  This specification (see Section 9.3.2).

   Applications that use this media type:

   Additional information:

      Magic number(s):  none

      File extension(s):  none

      Macintosh file type code(s):  none

   Person and email address to contact for further information:  See
      Authors Section.

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author/Change controller:  IESG

10.  Security Considerations

   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 as
   described by this document.  The discussion does not include
   definitive solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.

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10.1.  Personal Information

   HTTP clients are often privy to large amounts of personal information
   (e.g. the user's name, location, mail address, passwords, encryption
   keys, etc.), and SHOULD be very careful to prevent unintentional
   leakage of this information.  We very strongly recommend that a
   convenient interface be provided for the user to control
   dissemination of such information, and that designers and
   implementors be particularly careful in this area.  History shows
   that errors in this area often create serious security and/or privacy
   problems and generate highly adverse publicity for the implementor's

10.2.  Abuse of Server Log Information

   A server is in the position to save personal data about a user's
   requests which might identify their reading patterns or subjects of
   interest.  This information is clearly confidential in nature and its
   handling can be constrained by law in certain countries.  People
   using HTTP to provide data are responsible for ensuring that such
   material is not distributed without the permission of any individuals
   that are identifiable by the published results.

10.3.  Attacks Based On File and Path Names

   Implementations of HTTP origin servers SHOULD be careful to restrict
   the documents returned by HTTP requests to be only those that were
   intended by the server administrators.  If an HTTP server translates
   HTTP URIs directly into file system calls, the server MUST take
   special care not to serve files that were not intended to be
   delivered to HTTP clients.  For example, UNIX, Microsoft Windows, and
   other operating systems use ".." as a path component to indicate a
   directory level above the current one.  On such a system, an HTTP
   server MUST disallow any such construct in the Request-URI if it
   would otherwise allow access to a resource outside those intended to
   be accessible via the HTTP server.  Similarly, files intended for
   reference only internally to the server (such as access control
   files, configuration files, and script code) MUST be protected from
   inappropriate retrieval, since they might contain sensitive
   information.  Experience has shown that minor bugs in such HTTP
   server implementations have turned into security risks.

10.4.  DNS Spoofing

   Clients using HTTP rely heavily on the Domain Name Service, and are
   thus generally prone to security attacks based on the deliberate mis-
   association of IP addresses and DNS names.  Clients need to be
   cautious in assuming the continuing validity of an IP number/DNS name

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   In particular, HTTP clients SHOULD rely on their name resolver for
   confirmation of an IP number/DNS name association, rather than
   caching the result of previous host name lookups.  Many platforms
   already can cache host name lookups locally when appropriate, and
   they SHOULD be configured to do so.  It is proper for these lookups
   to be cached, however, only when the TTL (Time To Live) information
   reported by the name server makes it likely that the cached
   information will remain useful.

   If HTTP clients cache the results of host name lookups in order to
   achieve a performance improvement, they MUST observe the TTL
   information reported by DNS.

   If HTTP clients do not observe this rule, they could be spoofed when
   a previously-accessed server's IP address changes.  As network
   renumbering is expected to become increasingly common [RFC1900], the
   possibility of this form of attack will grow.  Observing this
   requirement thus reduces this potential security vulnerability.

   This requirement also improves the load-balancing behavior of clients
   for replicated servers using the same DNS name and reduces the
   likelihood of a user's experiencing failure in accessing sites which
   use that strategy.

10.5.  Proxies and Caching

   By their very nature, HTTP proxies are men-in-the-middle, and
   represent an opportunity for man-in-the-middle attacks.  Compromise
   of the systems on which the proxies run can result in serious
   security and privacy problems.  Proxies have access to security-
   related information, personal information about individual users and
   organizations, and proprietary information belonging to users and
   content providers.  A compromised proxy, or a proxy implemented or
   configured without regard to security and privacy considerations,
   might be used in the commission of a wide range of potential attacks.

   Proxy operators should protect the systems on which proxies run as
   they would protect any system that contains or transports sensitive
   information.  In particular, log information gathered at proxies
   often contains highly sensitive personal information, and/or
   information about organizations.  Log information should be carefully
   guarded, and appropriate guidelines for use developed and followed.
   (Section 10.2).

   Proxy implementors should consider the privacy and security
   implications of their design and coding decisions, and of the

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   configuration options they provide to proxy operators (especially the
   default configuration).

   Users of a proxy need to be aware that they are no trustworthier than
   the people who run the proxy; HTTP itself cannot solve this problem.

   The judicious use of cryptography, when appropriate, may suffice to
   protect against a broad range of security and privacy attacks.  Such
   cryptography is beyond the scope of the HTTP/1.1 specification.

10.6.  Denial of Service Attacks on Proxies

   They exist.  They are hard to defend against.  Research continues.

11.  Acknowledgments

   HTTP has evolved considerably over the years.  It has benefited from
   a large and active developer community--the many people who have
   participated on the www-talk mailing list--and it is that community
   which has been most responsible for the success of HTTP and of the
   World-Wide Web in general.  Marc Andreessen, Robert Cailliau, Daniel
   W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M.
   Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli,
   Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special
   recognition for their efforts in defining early aspects of the

   This document has benefited greatly from the comments of all those
   participating in the HTTP-WG.  In addition to those already
   mentioned, the following individuals have contributed to this

   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc
   Hedlund, Greg Herlihy, Koen Holtman, Alex Hopmann, Bob Jernigan, Shel
   Kaphan, Rohit Khare, John Klensin, Martijn Koster, Alexei Kosut,
   David M. Kristol, Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert
   Lunde, John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David
   Morris, Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott
   Powers, Owen Rees, Luigi Rizzo, David Robinson, Marc Salomon, Rich
   Salz, Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
   Simon E. Spero, Richard N. Taylor, Robert S. Thau, Bill (BearHeart)
   Weinman, Francois Yergeau, Mary Ellen Zurko, Josh Cohen.

   Thanks to the "cave men" of Palo Alto.  You know who you are.

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   Jim Gettys (the editor of [RFC2616]) wishes particularly to thank Roy
   Fielding, the editor of [RFC2068], along with John Klensin, Jeff
   Mogul, Paul Leach, Dave Kristol, Koen Holtman, John Franks, Josh
   Cohen, Alex Hopmann, Scott Lawrence, and Larry Masinter for their
   help.  And thanks go particularly to Jeff Mogul and Scott Lawrence
   for performing the "MUST/MAY/SHOULD" audit.

   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
   Frystyk implemented RFC 2068 early, and we wish to thank them for the
   discovery of many of the problems that this document attempts to

   This specification makes heavy use of the augmented BNF and generic
   constructs defined by David H. Crocker for [RFC5234].  Similarly, it
   reuses many of the definitions provided by Nathaniel Borenstein and
   Ned Freed for MIME [RFC2045].  We hope that their inclusion in this
   specification will help reduce past confusion over the relationship
   between HTTP and Internet mail message formats.

12.  References

12.1.  Normative References

              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
              Semantics", draft-ietf-httpbis-p2-semantics-05 (work in
              progress), November 2008.

   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
              and Content Negotiation", draft-ietf-httpbis-p3-payload-05
              (work in progress), November 2008.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-05 (work
              in progress), November 2008.

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   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
              draft-ietf-httpbis-p6-cache-05 (work in progress),
              November 2008.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              STD 66, January 2005.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

12.2.  Informative References

   [Kri2001]  Kristol, D., "HTTP Cookies: Standards, Privacy, and
              Politics", ACM Transactions on Internet Technology Vol. 1,
              #2, November 2001, <>.

   [Nie1997]  Nielsen, H., Gettys, J., Prud'hommeaux, E., Lie, H., and
              C. Lilley, "Network Performance Effects of HTTP/1.1, CSS1,
              and PNG", ACM Proceedings of the ACM SIGCOMM '97
              conference on Applications, technologies, architectures,
              and protocols for computer communication SIGCOMM '97,
              September 1997,

   [Pad1995]  Padmanabhan, V. and J. Mogul, "Improving HTTP Latency",
              Computer Networks and ISDN Systems v. 28, pp. 25-35,
              December 1995,

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application

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              and Support", STD 3, RFC 1123, October 1989.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC1436]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
              Torrey, D., and B. Alberti, "The Internet Gopher Protocol
              (a distributed document search and retrieval protocol)",
              RFC 1436, March 1993.

   [RFC1900]  Carpenter, B. and Y. Rekhter, "Renumbering Needs Work",
              RFC 1900, February 1996.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC2109]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2109, February 1997.

   [RFC2145]  Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
              and Interpretation of HTTP Version Numbers", RFC 2145,
              May 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC3977]  Feather, C., "Network News Transfer Protocol (NNTP)",
              RFC 3977, October 2006.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 115,

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              RFC 4395, February 2006.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC822]   Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC959]   Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [Spe]      Spero, S., "Analysis of HTTP Performance Problems",

   [Tou1998]  Touch, J., Heidemann, J., and K. Obraczka, "Analysis of
              HTTP Performance", ISI Research Report ISI/RR-98-463,
              Aug 1998, <>.

              (original report dated Aug. 1996)

   [WAIS]     Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T.,
              Wang, R., Sui, J., and M. Grinbaum, "WAIS Interface
              Protocol Prototype Functional Specification (v1.5)",
              Thinking Machines Corporation , April 1990.

Appendix A.  Tolerant Applications

   Although this document specifies the requirements for the generation
   of HTTP/1.1 messages, not all applications will be correct in their
   implementation.  We therefore recommend that operational applications
   be tolerant of deviations whenever those deviations can be
   interpreted unambiguously.

   Clients SHOULD be tolerant in parsing the Status-Line and servers
   tolerant when parsing the Request-Line.  In particular, they SHOULD
   accept any amount of SP or HTAB characters between fields, even
   though only a single SP is required.

   The line terminator for message-header fields is the sequence CRLF.
   However, we recommend that applications, when parsing such headers,
   recognize a single LF as a line terminator and ignore the leading CR.

   The character set of an entity-body SHOULD be labeled as the lowest
   common denominator of the character codes used within that body, with
   the exception that not labeling the entity is preferred over labeling
   the entity with the labels US-ASCII or ISO-8859-1.  See [Part3].

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   Additional rules for requirements on parsing and encoding of dates
   and other potential problems with date encodings include:

   o  HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
      which appears to be more than 50 years in the future is in fact in
      the past (this helps solve the "year 2000" problem).

   o  An HTTP/1.1 implementation MAY internally represent a parsed
      Expires date as earlier than the proper value, but MUST NOT
      internally represent a parsed Expires date as later than the
      proper value.

   o  All expiration-related calculations MUST be done in GMT.  The
      local time zone MUST NOT influence the calculation or comparison
      of an age or expiration time.

   o  If an HTTP header incorrectly carries a date value with a time
      zone other than GMT, it MUST be converted into GMT using the most
      conservative possible conversion.

Appendix B.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 3.3.1) to
   simplify the process of date comparison.  Proxies and gateways from
   other protocols SHOULD ensure that any Date header field present in a
   message conforms to one of the HTTP/1.1 formats and rewrite the date
   if necessary.

Appendix C.  Compatibility with Previous Versions

   HTTP has been in use by the World-Wide Web global information
   initiative since 1990.  The first version of HTTP, later referred to
   as HTTP/0.9, was a simple protocol for hypertext data transfer across
   the Internet with only a single method and no metadata.  HTTP/1.0, as
   defined by [RFC1945], added a range of request methods and MIME-like
   messaging that could include metadata about the data transferred and
   modifiers on the request/response semantics.  However, HTTP/1.0 did
   not sufficiently take into consideration the effects of hierarchical
   proxies, caching, the need for persistent connections, or name-based
   virtual hosts.  The proliferation of incompletely-implemented
   applications calling themselves "HTTP/1.0" further necessitated a
   protocol version change in order for two communicating applications
   to determine each other's true capabilities.

   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
   requirements that enable reliable implementations, adding only those

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   new features that will either be safely ignored by an HTTP/1.0
   recipient or only sent when communicating with a party advertising
   compliance with HTTP/1.1.

   It is beyond the scope of a protocol specification to mandate
   compliance with previous versions.  HTTP/1.1 was deliberately
   designed, however, to make supporting previous versions easy.  It is
   worth noting that, at the time of composing this specification
   (1996), we would expect commercial HTTP/1.1 servers to:

   o  recognize the format of the Request-Line for HTTP/0.9, 1.0, and
      1.1 requests;

   o  understand any valid request in the format of HTTP/0.9, 1.0, or

   o  respond appropriately with a message in the same major version
      used by the client.

   And we would expect HTTP/1.1 clients to:

   o  recognize the format of the Status-Line for HTTP/1.0 and 1.1

   o  understand any valid response in the format of HTTP/0.9, 1.0, or

   For most implementations of HTTP/1.0, each connection is established
   by the client prior to the request and closed by the server after
   sending the response.  Some implementations implement the Keep-Alive
   version of persistent connections described in Section 19.7.1 of

C.1.  Changes from HTTP/1.0

   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.

C.1.1.  Changes to Simplify Multi-homed Web Servers and Conserve IP

   The requirements that clients and servers support the Host request-
   header, report an error if the Host request-header (Section 8.4) is
   missing from an HTTP/1.1 request, and accept absolute URIs
   (Section 5.1.2) are among the most important changes defined by this

   Older HTTP/1.0 clients assumed a one-to-one relationship of IP

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   addresses and servers; there was no other established mechanism for
   distinguishing the intended server of a request than the IP address
   to which that request was directed.  The changes outlined above will
   allow the Internet, once older HTTP clients are no longer common, to
   support multiple Web sites from a single IP address, greatly
   simplifying large operational Web servers, where allocation of many
   IP addresses to a single host has created serious problems.  The
   Internet will also be able to recover the IP addresses that have been
   allocated for the sole purpose of allowing special-purpose domain
   names to be used in root-level HTTP URLs.  Given the rate of growth
   of the Web, and the number of servers already deployed, it is
   extremely important that all implementations of HTTP (including
   updates to existing HTTP/1.0 applications) correctly implement these

   o  Both clients and servers MUST support the Host request-header.

   o  A client that sends an HTTP/1.1 request MUST send a Host header.

   o  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
      request does not include a Host request-header.

   o  Servers MUST accept absolute URIs.

C.2.  Compatibility with HTTP/1.0 Persistent Connections

   Some clients and servers might wish to be compatible with some
   previous implementations of persistent connections in HTTP/1.0
   clients and servers.  Persistent connections in HTTP/1.0 are
   explicitly negotiated as they are not the default behavior.  HTTP/1.0
   experimental implementations of persistent connections are faulty,
   and the new facilities in HTTP/1.1 are designed to rectify these
   problems.  The problem was that some existing 1.0 clients may be
   sending Keep-Alive to a proxy server that doesn't understand
   Connection, which would then erroneously forward it to the next
   inbound server, which would establish the Keep-Alive connection and
   result in a hung HTTP/1.0 proxy waiting for the close on the
   response.  The result is that HTTP/1.0 clients must be prevented from
   using Keep-Alive when talking to proxies.

   However, talking to proxies is the most important use of persistent
   connections, so that prohibition is clearly unacceptable.  Therefore,
   we need some other mechanism for indicating a persistent connection
   is desired, which is safe to use even when talking to an old proxy
   that ignores Connection.  Persistent connections are the default for
   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
   declaring non-persistence.  See Section 8.1.

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   The original HTTP/1.0 form of persistent connections (the Connection:
   Keep-Alive and Keep-Alive header) is documented in [RFC2068].

C.3.  Changes from RFC 2068

   This specification has been carefully audited to correct and
   disambiguate key word usage; RFC 2068 had many problems in respect to
   the conventions laid out in [RFC2119].

   Transfer-coding and message lengths all interact in ways that
   required fixing exactly when chunked encoding is used (to allow for
   transfer encoding that may not be self delimiting); it was important
   to straighten out exactly how message lengths are computed.
   (Sections 3.4, 4.4, 8.2, see also [Part3], [Part5] and [Part6])

   The use and interpretation of HTTP version numbers has been clarified
   by [RFC2145].  Require proxies to upgrade requests to highest
   protocol version they support to deal with problems discovered in
   HTTP/1.0 implementations (Section 3.1)

   Transfer-coding had significant problems, particularly with
   interactions with chunked encoding.  The solution is that transfer-
   codings become as full fledged as content-codings.  This involves
   adding an IANA registry for transfer-codings (separate from content
   codings), a new header field (TE) and enabling trailer headers in the
   future.  Transfer encoding is a major performance benefit, so it was
   worth fixing [Nie1997].  TE also solves another, obscure, downward
   interoperability problem that could have occurred due to interactions
   between authentication trailers, chunked encoding and HTTP/1.0
   clients.(Section 3.4, 3.4.1, and 8.5)

C.4.  Changes from RFC 2616

   Rules about implicit linear white space between certain grammar
   productions have been removed; now it's only allowed when
   specifically pointed out in the ABNF.  The CHAR rule does not allow
   the NUL character anymore (this affects the comment and quoted-string
   rules).  Furthermore, the quoted-pair rule does not allow escaping
   NUL, CR or LF anymore.  (Section 2.2)

   Clarify that HTTP-Version is case sensitive.  (Section 3.1)

   Remove reference to non-existant identity transfer-coding value
   tokens.  (Sections 3.4 and 4.4)

   Clarification that the chunk length does not include the count of the
   octets in the chunk header and trailer.  (Section 3.4.1)

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   Update use of abs_path production from RFC1808 to the path-absolute +
   query components of RFC3986.  (Section 5.1.2)

   Clarify exactly when close connection options must be sent.
   (Section 8.1)

Appendix D.  Terminology

   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, the HTTP communication.


      A transport layer virtual circuit established between two programs
      for the purpose of communication.


      The basic unit of HTTP communication, consisting of a structured
      sequence of octets matching the syntax defined in Section 4 and
      transmitted via the connection.


      An HTTP request message, as defined in Section 5.


      An HTTP response message, as defined in Section 6.


      A network data object or service that can be identified by a URI,
      as defined in Section 3.2.  Resources may be available in multiple
      representations (e.g. multiple languages, data formats, size, and
      resolutions) or vary in other ways.


      The information transferred as the payload of a request or
      response.  An entity consists of metainformation in the form of
      entity-header fields and content in the form of an entity-body, as
      described in Section 4 of [Part3].


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      An entity included with a response that is subject to content
      negotiation, as described in Section 5 of [Part3].  There may
      exist multiple representations associated with a particular
      response status.

   content negotiation

      The mechanism for selecting the appropriate representation when
      servicing a request, as described in Section 5 of [Part3].  The
      representation of entities in any response can be negotiated
      (including error responses).


      A resource may have one, or more than one, representation(s)
      associated with it at any given instant.  Each of these
      representations is termed a `variant'.  Use of the term `variant'
      does not necessarily imply that the resource is subject to content


      A program that establishes connections for the purpose of sending

   user agent

      The client which initiates a request.  These are often browsers,
      editors, spiders (web-traversing robots), or other end user tools.


      An application program that accepts connections in order to
      service requests by sending back responses.  Any given program may
      be capable of being both a client and a server; our use of these
      terms refers only to the role being performed by the program for a
      particular connection, rather than to the program's capabilities
      in general.  Likewise, any server may act as an origin server,
      proxy, gateway, or tunnel, switching behavior based on the nature
      of each request.

   origin server

      The server on which a given resource resides or is to be created.


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      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.
      Requests are serviced internally or by passing them on, with
      possible translation, to other servers.  A proxy MUST implement
      both the client and server requirements of this specification.  A
      "transparent proxy" is a proxy that does not modify the request or
      response beyond what is required for proxy authentication and
      identification.  A "non-transparent proxy" is a proxy that
      modifies the request or response in order to provide some added
      service to the user agent, such as group annotation services,
      media type transformation, protocol reduction, or anonymity
      filtering.  Except where either transparent or non-transparent
      behavior is explicitly stated, the HTTP proxy requirements apply
      to both types of proxies.


      A server which acts as an intermediary for some other server.
      Unlike a proxy, a gateway receives requests as if it were the
      origin server for the requested resource; the requesting client
      may not be aware that it is communicating with a gateway.


      An intermediary program which is acting as a blind relay between
      two connections.  Once active, a tunnel is not considered a party
      to the HTTP communication, though the tunnel may have been
      initiated by an HTTP request.  The tunnel ceases to exist when
      both ends of the relayed connections are closed.


      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion.  A
      cache stores cacheable responses in order to reduce the response
      time and network bandwidth consumption on future, equivalent
      requests.  Any client or server may include a cache, though a
      cache cannot be used by a server that is acting as a tunnel.


      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests.
      The rules for determining the cacheability of HTTP responses are
      defined in Section 1 of [Part6].  Even if a resource is cacheable,
      there may be additional constraints on whether a cache can use the
      cached copy for a particular request.

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      Upstream and downstream describe the flow of a message: all
      messages flow from upstream to downstream.


      Inbound and outbound refer to the request and response paths for
      messages: "inbound" means "traveling toward the origin server",
      and "outbound" means "traveling toward the user agent"

Appendix E.  Change Log (to be removed by RFC Editor before publication)

E.1.  Since RFC2616

   Extracted relevant partitions from [RFC2616].

E.2.  Since draft-ietf-httpbis-p1-messaging-00

   Closed issues:

   o  <>: "HTTP Version
      should be case sensitive"

   o  <>: "'unsafe'
      characters" (<>)

   o  <>: "Chunk Size
      Definition" (<>)

   o  <>: "Message Length"

   o  <>: "Media Type
      Registrations" (<>)

   o  <>: "URI includes
      query" (<>)

   o  <>: "No close on
      1xx responses" (<>)

   o  <>: "Remove
      'identity' token references"

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   o  <>: "Import query

   o  <>: "qdtext BNF"

   o  <>: "Normative and
      Informative references"

   o  <>: "RFC2606

   o  <>: "RFC977

   o  <>: "RFC1700

   o  <>: "inconsistency
      in date format explanation"

   o  <>: "Date reference

   o  <>: "Informative

   o  <>: "ISO-8859-1

   o  <>: "Normative up-
      to-date references"

   Other changes:

   o  Update media type registrations to use RFC4288 template.

   o  Use names of RFC4234 core rules DQUOTE and HTAB, fix broken ABNF
      for chunk-data (work in progress on

E.3.  Since draft-ietf-httpbis-p1-messaging-01

   Closed issues:

   o  <>: "Bodies on GET
      (and other) requests"

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   o  <>: "Updating to

   o  <>: "Status Code
      and Reason Phrase"

   o  <>: "rel_path not

   Ongoing work on ABNF conversion

   o  Get rid of duplicate BNF rule names ("host" -> "uri-host",
      "trailer" -> "trailer-part").

   o  Avoid underscore character in rule names ("http_URL" -> "http-
      URL", "abs_path" -> "path-absolute").

   o  Add rules for terms imported from URI spec ("absoluteURI",
      "authority", "path-absolute", "port", "query", "relativeURI",
      "host) -- these will have to be updated when switching over to

   o  Synchronize core rules with RFC5234 (this includes a change to
      CHAR which now excludes NUL).

   o  Get rid of prose rules that span multiple lines.

   o  Get rid of unused rules LOALPHA and UPALPHA.

   o  Move "Product Tokens" section (back) into Part 1, as "token" is
      used in the definition of the Upgrade header.

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

   o  Rewrite prose rule "token" in terms of "tchar", rewrite prose rule

E.4.  Since draft-ietf-httpbis-p1-messaging-02

   Closed issues:

   o  <>: "HTTP-date vs.

   o  <>: "WS in quoted-

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   Ongoing work on IANA Message Header Registration

   o  Reference RFC 3984, and update header registrations for headers
      defined in this document.

   Ongoing work on ABNF conversion

   o  Replace string literals when the string really is case-sensitive

E.5.  Since draft-ietf-httpbis-p1-messaging-03

   Closed issues:

   o  <>: "Connection

   o  <>: "Move
      registrations and registry information to IANA Considerations"

   o  <>: "need new URL
      for PAD1995 reference"

   o  <>: "IANA
      Considerations: update HTTP URI scheme registration"

   o  <>: "Cite HTTPS
      URI scheme definition"

   o  <>: "List-type
      headers vs Set-Cookie"

   Ongoing work on ABNF conversion

   o  Replace string literals when the string really is case-sensitive

   o  Replace HEX by HEXDIG for future consistence with RFC 5234's core

E.6.  Since draft-ietf-httpbis-p1-messaging-04

   Closed issues:

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   o  <>: "Out-of-date
      reference for URIs"

   o  <>: "RFC 2822 is
      updated by RFC 5322"

   Ongoing work on ABNF conversion

   o  Use "/" instead of "|" for alternatives.

   o  Get rid of RFC822 dependency; use RFC5234 plus extensions instead.

   o  Only reference RFC 5234's core rules.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      value format definitions.


      application/http Media Type  45

      cache  60
      cacheable  60
      client  59
      connection  58
      Connection header  35
      content negotiation  59
      Content-Length header  36

      Date header  36
      downstream  61

      entity  58

      gateway  60
         absolute-URI  12
         asctime-date  14

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         attribute  16
         authority  12
         BWS  9
         chunk  17
         chunk-data  17
         chunk-ext  17
         chunk-ext-name  17
         chunk-ext-val  17
         chunk-size  17
         Chunked-Body  17
         comment  10
         Connection  35
         connection-token  35
         Connection-v  35
         Content-Length  36
         Content-Length-v  36
         ctext  10
         Date  36
         Date-v  36
         date1  14
         date2  14
         date3  14
         extension-code  27
         extension-method  24
         field-content  20
         field-name  20
         field-value  20
         general-header  23
         generic-message  19
         Host  38
         Host-v  38
         HTTP-date  14
         HTTP-message  19
         HTTP-Prot-Name  11
         http-URI  13
         HTTP-Version  11
         last-chunk  17
         message-body  21
         message-header  20
         Method  24
         month  14
         obsolete-date  14
         OWS  9
         parameter  16
         path-absolute  12
         port  12
         product  18
         product-version  18

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         protocol-name  42
         protocol-version  42
         pseudonym  42
         qdtext  10
         query  12
         quoted-pair  10
         quoted-string  10
         quoted-text  10
         Reason-Phrase  27
         received-by  42
         received-protocol  42
         relative-part  12
         relativeURI  12
         Request  24
         Request-Line  24
         Request-URI  24
         Response  26
         rfc850-date  14
         rfc1123-date  14
         RWS  9
         start-line  19
         Status-Code  27
         Status-Line  27
         t-codings  38
         tchar  10
         TE  38
         TE-v  38
         TEXT  9
         time  14
         token  10
         Trailer  40
         trailer-part  17
         Trailer-v  40
         transfer-coding  16
         Transfer-Encoding  40
         Transfer-Encoding-v  40
         transfer-extension  16
         Upgrade  41
         Upgrade-v  41
         uri-host  12
         URI-reference  12
         value  16
         Via  42
         Via-v  42
         weekday  14
         wkday  14


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         Connection  35
         Content-Length  36
         Date  36
         Host  38
         TE  38
         Trailer  39
         Transfer-Encoding  40
         Upgrade  41
         Via  42
      Host header  38
      http URI scheme  13
      https URI scheme  13

      inbound  61

      Media Type
         application/http  45
         message/http  44
      message  58
      message/http Media Type  44

      origin server  59
      outbound  61

      proxy  59

      representation  58
      request  58
      resource  58
      response  58

      server  59

      TE header  38
      Trailer header  39
      Transfer-Encoding header  40
      tunnel  60

      Upgrade header  41

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      upstream  61
      URI scheme
         http  13
         https  13
      user agent  59

      variant  59
      Via header  42

Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305

   Jim Gettys
   One Laptop per Child
   21 Oak Knoll Road
   Carlisle, MA  01741


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304


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   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052


   Larry Masinter
   Adobe Systems, Incorporated
   345 Park Ave
   San Jose, CA  95110


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139


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   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155

   Phone: +49 251 2807760
   Fax:   +49 251 2807761

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

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Fielding, et al.          Expires May 20, 2009                 [Page 72]